Riichi Mahjong:All Simples
All Simples, also widely known as Tan'yao (Japanese:
All simples is an extremely common yaku in riichi mahjong. The occurrence rate of this yaku is the second highest among all yakus, right after riichi.
A winning hand contains this yaku when it has no terminals or honors.
For the same reason, all simples is preferred and welcomed by newbies. Though it's easy to make a hand of "Tanyao nomi(タンヤオのみtan'yao nomi, only tan'yao)", it's not that frequent to see in high-level tables for its low cost-effectiveness.
All simples is also frequently used when one wants to end another's dealer or block others' high-cost hands. There's this saying in China, "No matter how good your hand is, a tan'yao will end it all(功夫再高，也怕断幺)".